The present invention relates to electronic devices and, more particularly, to electronic devices for igniting and driving fluorescent lamps.
A conventional fluorescent lamp includes a partially evacuated glass tube containing an inert gas at low pressure with a fluorescent coating on its inner surface, and electrodes at each end. The electrodes are conventionally thermionic filaments which are heated during the starting process to provide a supply of electrons for initiating a gas-discharge current which ionizes the gas in the tube. A ballast reactor is disposed in the supply line to the electrodes. A thermally controlled breaker periodically shorts the filaments whereby the heating current is applied thereto. After a short heating period, the breaker opens. A collapsing magnetic field in the reactor produces a high-voltage spike capable of initiating current flow through the gas between the electrodes. The resulting gas-discharge current flow generates ultra-violet emissions. The ultra-violet emissions stimulate fluorescence in the fluorescent coating on the inside of the tube, thereby producing visible light. The tube is made of a material such as, for example, soda glass, which blocks the transmission of ultra-violet emissions therethrough and prevents possible harm to objects external to the tube.
Once current begins to flow between the electrodes, the resistance therebetween drops to a fraction of its previous value. The ballast reactor limits the current through the tube. The reduced resistance through the tube substantially reduces the voltage drop therethrough and effectively maintains the starting circuit and the filaments deenergized.
When the fluorescent lamp is driven by an AC source operating at, for example, about 60 Hz, current flow through the tube is extinguished each time the source voltage passes through zero. Thus, such a fluorescent lamp is effectively extinguished for a short time and then re-ignited at a frequency of 120 Hz. Residual ionization in the tube permits current to become re-established without requiring the operation of the starting circuit.
Due to the relatively low line frequency at which it operates, the ionization of the gas in the tube tends to decay between its energized periods. A substantial portion of a cycle is required to re-establish current flow through the gas. While awaiting the resumption of current flow, the light output of the lamp is degraded. Furthermore, the ions in the gas are accelerated toward the electrodes during this time and tend to blacken the tube in their vicinity and reduce the life of the filaments. Also, the relatively low frequency of the conventional AC source require a large and expensive ballast inductor.
Battery operation of a fluorescent lamp is a convenient option. One type of conventional battery operation employs a flyback system for producing periodic narrow unipolar pulses which are applied across the electrodes in the lamp. The output of the flyback system, consisting of unidirectional pulses, energizes the lamp for conduction in only a single direction. Such unidirectional conduction is effective for accelerating the ions toward the electrodes and can effectively destroy the electrodes in a short time.